(1) Field of the Invention
The present invention relates to a plasma-display panel used for a computer and a TV, and a driving method of such a plasma-display panel.
(2) Related Art
In recent years, upsizing and higher definition of display devices used for computers and TVs are in high demand. Hopes run high that plasma-display panels (PDP) will come up to such expectations for their thinness and lightness.
There are two types of PDPs: DC-PDPs and AC-PDPs.
FIG. 1 shows a schematic representation of a conventional DC-PDP. On the upper surface of glass plate 11 used as a back plate, anode line electrode group 12a and auxiliary line electrode group 12b are arranged in parallel. Thick film resistance 13, which is a discharge electrode limiting element, branches from each line electrode. Insulator layer 14 is deposited over anode line electrode group 12a, auxiliary line electrode group 12b, and thick film resistance 13. Insulator layer 14 has through holes. The interior surface of each through hole is placed with electrode pad 15 connected to a terminal of each thick film resistance 13.
On the surface of insulator layer 14, partitions 16 are arranged so as to form discharge cells 20 and auxiliary cells 20a. In each discharge cell 20, a phosphor layer 19 is arranged on the side and the bottom.
On the lower surface of glass plate 18 used as a front plate, cathode line electrode group 17 is arranged.
Anode line electrode group 12a and electrode pad 15 are exposed in discharge cell 20, and auxiliary line electrode group 12b is exposed in auxiliary cell 20a.
FIG. 2 shows a matrix layout circuit of the DC-PDP.
Horizontally, reset cathode line R is set as the first line, followed by cathode line electrodes K.sub.1 -K.sub.N. Vertically, anode line electrodes A.sub.1 -A.sub.M, and auxiliary line electrodes H.sub.1 -H.sub.L are set.
FIG. 3 is a time chart which shows timing of applying pulses to each electrode. This chart relates to a pulse memory method which has been conventionally used for the DC-PDPs. First, addressing is carried out: while scanning cathode line electrodes K.sub.1 -K.sub.N, electrical charges are generated by pulse discharge in the discharge cell (display cell) which should be lit up. After that, the discharge is sustained. However, as the electrical charges remain only for a short period of time, they cannot store a screen of image information. In order to cope with this problem, the following method is used.
First, in scanning period t.sub.1, several pulses of opposite phase are simultaneously applied to auxiliary anode group H.sub.1 -H.sub.L and reset cathode line R, thereby generating a stable reset discharge.
Next, in scanning period t.sub.3 in which the charged particles generated by the reset discharge remain, by applying a pulse to the auxiliary line electrode group H.sub.1 -H.sub.L and the first cathode line electrode K.sub.1, and a write pulse to the electrodes corresponding to the display cell in anode line electrode group A.sub.1 -A.sub.M, stable auxiliary discharge occurs between auxiliary line electrode group H.sub.1 -H.sub.L and cathode line electrode K.sub.1. This is ignited by the remaining charged particles. Moreover, being ignited by the auxiliary discharge, stable main discharge occurs between cathode line electrode K.sub.1 and electrodes corresponding to the display cell in anode electrode group A.sub.1 -A.sub.M.
The main discharge in the display cell is sustained by: generating a main discharge in the display cell by applying a sustain pulse to cathode line electrode K.sub.1 in scanning period t.sub.6, in which much of the charged particles generated by the main discharge in scanning period t.sub.3 remain; and by doing the same in scanning periods t.sub.8, t.sub.10, . . .
Next, in scanning period t.sub.5, in which the charged particles generated by the auxiliary discharge in scanning period t.sub.3 remain and a sustain pulse is not applied, by applying a pulse to auxiliary line electrode group H.sub.1 -H.sub.L and the second cathode line electrode K.sub.2, and a write pulse to electrodes corresponding to the display cell in anode line electrode group A.sub.1 -A.sub.M, stable auxiliary discharge takes place between auxiliary line electrode group H.sub.1 -H.sub.L and cathode line electrode K.sub.2. This is ignited by the remaining charged particles. Being ignited by the auxiliary discharge, stable main discharge occurs between cathode line electrode K.sub.2 and electrodes corresponding to the display cell in the group of anode line electrodes A.sub.1 -A.sub.M.
The main discharge in the display cell is sustained by: generating the main discharge by applying a sustain pulse to cathode line electrode K.sub.2 in scanning period t.sub.8, in which much of the charged particles generated by the main discharge in scanning period t.sub.5 remain; and by doing the same in scanning periods t.sub.10, t.sub.12, . . .
The above mentioned auxiliary discharge and the main discharge is carried out in the same way with regard to cathode line electrode K.sub.3, . . . K.sub.N in scanning periods t.sub.7 . . . , thus forming a screen of image.
How to display TV images in the pulse memory method of the DC-PDP-mentioned above can be explained as follows.
In NTSC system, a TV image is composed of 60 fields per second. PDPs can only show two-level graduation by "ON" and "OFF". Tones in-between are displayed as follows. For red (R), green (G), and blue (B), respectively, one field is divided into several sub-fields and "ON" time is timeshared. Tones in-between "ON" and "OFF" are displayed by the combination of the sub-fields. This method is called "field timesharing graduation display method".
FIG. 4 is a graph showing the field dividing method for 256 gray scales. The horizontal axis shows time and the vertical axis shows order of the scanning lines (scans from top to bottom), the slashed part represents discharge sustaining periods.
One field consists of eight sub-fields, each having an equal cycle. Write scanning is carried out in the cycle which is equal to a sub-field cycle. In each scanning line, discharge sustaining operation is carried out subsequent to the write scanning.
The ratio of the discharge sustaining period of each sub-field is set as 1, 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, and 1/128. By the combination of the eight bit binary, 256 gray scales can be displayed.
As is apparent from FIG. 4, the ratio of the discharge sustaining period in one field is only about 1/4. Moreover, as the sustain pulses are applied on and off, the ratio of the discharge sustaining period that really contributes to the emission is even smaller than the above mentioned ratio.
For example, when a cycle of the sustain pulse is 4 (.mu.sec), duty ratio of the sustain pulse is 1/2 of the theoretical maximum value, and the number of the sustain pulse applied to the shortest discharge sustaining period (1/128) is 3, the number of pulses per field (P) can be expressed as P=(2.sup.8 -1)* 3=765. The real discharge sustaining period in one field cycle (1/60 sec=16.67 msec) becomes 4*1/2*765(.mu.sec)=1.53(msec). In this case, the discharge sustaining period which really contributes to the emission is less than 10% of a field cycle.
As mentioned above, as the conventional DC-PDPs have practically short discharge sustaining period, the maximum luminance can be around 150 cd/m.sup.2.
In the driving method mentioned above, as the addressing discharge is carried out in the short period of time between the sustain pulses, rise time of the addressing discharge pulse has to be short. For that reason, main discharge and sustaining of the discharge are caused by the effect of the residual charged particles generated by the auxiliary discharge of the preceding scanning line in each scanning line.
In such a case, if three discharge cells of red (R), green (G), and blue (B) are arranged on a scanning line, the dot size of one pixel becomes rather large. Therefore, good image quality for computers and TVs cannot be obtained. In order to cope with this problem, two scanning lines are used for one pixel as shown in FIG. 1. In FIG. 1, a red cell and a green cell are arranged on the upper line, and a green cell and a blue cell are arranged on the lower line. Even so, the real horizontal resolution is low and "white" cannot be displayed by one scanning line, which is not suitable for computer displays as they require high-definition image quality.
Also, when driving the DC-PDPs by the pulse memory method, as charging and discharging are repeated by applying pulses of several hundred volts to the capacitative load between the electrodes, electricity that does not directly contribute to the emission is consumed in a large amount. It is highly demanded that such reactive power should be reduced for energy saving.
In the case of conventional AC-PDPs, electrodes covered with dielectric layers are arranged. By accumulating the electric charges caused by the addressing discharge on the dielectric layer as wall charges, a screen of image information can be stored. Therefore, it is possible to apply the sustain pulse to all the scanning lines at a dash when the discharge is sustained. Therefore, the ratio of the discharge sustaining period in one field can be increased compared to the DC-PDP, but as the applied sustain pulse is AC, the emission in the discharge sustaining period is on and off. The real discharge sustaining period that contributes to the emission is up to 20-30% of one field cycle.
Also, the AC-PDPs have larger capacity (Cp) between electrodes compared to the DC-PDPs. Therefore, the amount of reactive power is large.
For example, in the case of a 21-type color monitor panel having 640(*3)pixels*480pixels, the entire capacity Cp could reach 17 nF.
When 17 nF is Cp, 180V is sustaining voltage V.sub.S, and 200 kH.sub.Z is frequency f, electricity Wc for driving in the discharge sustaining period is 2(charging/discharging)*2(inversion)*1/2*17 nF*(180V).sup.2 *200 kH.sub.Z =220 W.
Japanese Laid-open Patent Application No. 63-101897 discloses a method of suppressing the reactive power. It is to recover the reactive power by using inductance between the switching element in the driving circuit and the capacitative load and using the principle of the LC serial resonance circuit.
However, as can be understood from the fact that the condition of the plasma and the conduction rate differs greatly depending on the condition whether all the dots are lit up, or whether they are put out, capacitative load of the PDP applied to the driving circuit in the discharge sustaining period differs greatly when displaying motion pictures on TVs. Therefore, if the time constant of the serial resonance circuit of the driving circuit is constant, reactive power cannot be reduced so much.
Here, it possible to dynamically change the time constant depending on the discharge current of the panel in the discharge sustaining period. However, in that case, construction of the driving circuit becomes complicated and the cost increases greatly.